avformat/utils: dont count attached pics toward the probesize

Such pics behave more like headers which we also dont count.
Fixes Ticket3146

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit a8dec360c5db15e8da4b44ff3c0f02a6c57e8ac0)
build: avoid stdin stall with GNU AS probing.

a758c5e added probing for various tools, such as AS. Unfortunately, GNU
AS is reading stdin with -v, and thus configure is stalled with
configure arguments such as --as=as.

Fixes Ticket #1898.
(cherry picked from commit dbb41f93c16cbc65a899a75723c95da51c851cd5)